BMWs may be renowned for their engines and handling, but the electrical system helps deliver a lot of the features that make it a luxury vehicle. Along with firing the spark plugs and powering the headlights, it also lets you enjoy some music on your infotainment system while stuck in Baltimore traffic, regulates the speed with the cruise control while making the trek to Washington, and lets you tweak the steering and suspension settings to fit conditions ranging from scenic drives to potholes. Understanding how these systems work can help you find the parts you need to replace to get your BMW driving like a BMW again.
How Does the Electrical System Work?
Pretty much everything outside of the engine, steering and brakes is powered by electricity, and even these systems have electronic components.
This electricity comes from an alternator. This device combines an AC generator, a rectifier and a voltage regulator. As the engine runs, power is transferred from the crankshaft to the alternator pulley via a belt. As the pulley spins, the shaft connected to it spins a magnetic rotor past the stators, generating electricity. This is fed through the rectifier, which turns that AC power into DC power, then a voltage regulator ensures the electricity entering the electrical system is at the correct voltage.
Have a hybrid? Instead of an alternator, one of the electric motors acts as a generator when it's not moving the vehicle down the road. It generates high voltage power to recharge the hybrid battery, and some of that power is converted into low voltage power for the rest of the devices connected to the electrical system. Have an i3? The range extender's engine only powers a generator, with all powertrain duties handled by the electric motor. As for the electrical system, everything works the same as a hybrid with high voltage power converted to lower voltage for on-board systems.
Power is sent to the distribution box under the hood. This box contains the main fuses and relays for the vehicle. From there, the electrical system is divided into under hood components like the battery and ignition system, and body components including the lights and infotainment system. The distribution box is connected to the fuse box inside the cabin of your vehicle, which is where most of the body connections start. Some components bridge the gap between both sides: the headlights are controlled from the cabin directly and through a light sensor, while the lights are on the corners of the vehicle.
When the engine is off, power is provided by the battery. In hybrids, there's still a 12 volt battery on-board to power accessories. The battery is rated with Reserve Capacity Minutes (RCM,) the time the battery is expected to last powering accessories, and Cranking Amps (CA,) the amount of power available in a single burst to turn the starter motor.
If the alternator starts to fail, the battery will take up the slack, powering accessories until it's flat. With the added demand of the ignition system and other components, the total time a battery will last while the car is being driven will be far less than it would be if the engine was off and it was just powering the lights and infotainment system. If the alternator's voltage regulator fails, it could be sending uneven current with frequent power spikes, causing unusual behavior of electrical components and possibly blown fuses or damage to these components.
Electrical System Accessories
Most electric-powered systems in your BMW are made up of four main components: an input, an output, a control system and a set of wiring to connect the three. For example, in the air bag system, the input is the sensors around the vehicle perimeter and in the seats, the output is the airbags, and in between these, there's a control unit that deploys the correct airbags at the right time, depending on which sensors were triggered. If front sensor is triggered, the driver, passenger or both front air bags are fired depending which ones are occupied, while the correct side curtain airbag will fire if either the left or right impact sensor is triggered.
Sometimes this input/control/output system crosses several systems. The gauge cluster in your BMW needs to receive information from the engine, transmission and wheel sensors to show you the current speed and RPM, the controls inside the cabin to indicate turn signals, lights and other functions, and the fuel tank sensor to show how much fuel you have.
Finding Electrical Components
We use BMW's factory cataloging system and diagrams, which has a few idiosyncrasies. Understanding how they categorize parts can make it a lot easier to find what you're looking for.
When components cross several systems, BMW separates these parts into sections with specific functions. If you need a headlight housing, it can be found in the headlight section. If you need a headlight switch, you'll find it in the Instruments & Gauges section if it's dash-mounted or the Switches section if the switch is on a control stalk. Stand-alone systems like the keyless entry system list all the components together from control modules to ground cables.
Since you'll probably be replacing other related parts, BMW groups systems together, even if everything isn't electronic. While the only electronic component in a seat belt is the sensor in the buckle that tells the car that the belt has been fastened, the seat belt section lists all the seat belt components including the belts and assembly parts. Likewise, other categories like the wipers and sunroofs will show you all the parts of that system, whether they're mechanical, electrical or body components.
There are a lot of package and options available on these cars, which means you may be faced with several options when choosing a part; even valve stems are offered in different colors. Entering your vehicle information or the VIN can filter results so you only see components that may have been installed on your vehicle. From there, it's just a matter of checking the parts descriptions to find the part that matches the equipment on your specific BMW.
Getting Electrical Components for Your Car
GoBMWParts.com is your source for everything BMW. We only carry OEM parts, which means anything you order will be designed and built by BMW specifically for your car or crossover, maintaining factory quality and performance. They're even backed with a warranty from the automaker. Our site lets you narrow down results by entering your vehicle model and VIN, and lets you search for parts by part number and keywords like “wiring harness.” Still not sure what you need? Contact us to talk to our experienced parts personnel: they can help you find the right replacement parts to restore your BMW's performance.